A thermochemical fuel generating method is a technology of producing a chemical fuel from thermal energy that may be obtained from sunlight and the like, and of storing the thermal energy as a chemical fuel. The fuel is produced by a two-step thermal cycle (thermochemical cycle) which includes a step performed under a high temperature (first temperature) and a step performed under a low temperature (second temperature). In practice, when a catalyst oxide is reduced at a high temperature, and carbon dioxide that is a raw material gas, water vapor, and the like are introduced to the reduced catalyst oxide, the catalyst oxide absorbs oxygen from a raw material, and thus fuels such as a syngas, methane, hydrocarbon, alcohol, and hydrogen may be produced.
As oxide-based catalysts, mainly, ZnO—Zn, Fe2O3—FeO, CeO2—Ce2O3, CeO2 having a nonstoichiometric composition, mixtures thereof, a partially substituted oxide, and the like have been reported. Water vapor modification of methane (CH4+H2O→6H2+CO) using LaSrMnO3-based perovskite oxide has been reported, but this is completely different from thermochemical fuel generation (H2O→H2+½O2, or CO2→CO+½O2) using water or carbon dioxide, and thermochemical fuel production using a perovskite oxide AXO3 has not been reported until now.
Among fuels that may be generated by the thermochemical fuel generation method, for example, hydrogen is a clean energy source that generates only water after combustion, and thus hydrogen is expected as renewable energy.
When hydrogen is produced by directly decomposing water (H2O→H2+½O2), a high temperature of several thousands of ° C. is necessary, but when using the thermochemical fuel generation method, hydrogen may be produced by decomposing water by a thermal cycle of two-step temperature of relatively low temperature (for example, refer to Japanese unexamined Patent Application, First Publication No. 2004-269296).
In the two-step thermal cycle (thermochemical cycle) which includes a step performed under a high temperature (first temperature) and a step performed under a low temperature (second temperature), with respect to the high temperature heating, a technology using solar energy is known (for example, Japanese Unexamined Patent Application, First Publication No. 2009-263165).
Solar energy is, by far, the most abundant renewable energy source. To fully take advantage of this vast energy source, it must be efficiently stored in a stable form on a massive scale. To store sunlight into chemical forms, solar thermochemical fuel production using nonstoichiometric oxide has been researched. It is a two-step thermochemical cycle that drives redox reaction between the oxide and gas species. The oxide is reduced at a high temperature at which oxygen is released from the oxide. Then, at a lower temperature at which carbon dioxide and/or water vapor are introduced, the oxide strips oxygen atoms from the introduced gas. As a result, syngas, methane, and hydrogen fuels are produced. The thermodynamic efficiency is calculated to be 15 to 75% depending on the oxide systems including ZnO—Zn, Fe2O3—FeO, CeO2—Ce2O3, nonstoichiometric CeO2 systems, and some combinations between them. Other systems have been largely unexplored. A record solar-fuel conversion efficiency is 0.8% in a solar-thermochemical cycle of 800 to 1,630° C. using undoped ceria, with 1.3 to 1.5 liters of carbon monoxide and hydrogen production.
As catalyst oxides that are used in the thermochemical fuel generation method, cerium oxide (ceria) is known as indicated in the specification of US Patent Application Publication No. 2009/0107044. In the solar thermochemical efficiency experiment using undoped ceria, the solar reactor lost energy of 50% or less as heat, specifically above 1,250° C., and energy of 40% or less as solar re-reflection from the aperture. Thus, the large improvement in solar-fuel conversion efficiency can be anticipated. To address this issue, a mechanical engineering approach and a materials science approach are possible. A heat recovery system might also be integrated. The challenge in this route is how to choose an appropriate oxide structure as well as materials chemistry process from millions of candidate oxides that might show the desired properties. Combinatorial synthesis might be very useful to make candidate oxides, but a rapid way to check fuel productivity at high temperatures is required.